This invention relates to swimming pool alarm systems and more particularly to that type of system where the detection means is an immersed pressure transducer.
Prior art swimming pool alarm systems have generally been of two types: those having electrical contacts made or broken as a result of physical movement caused by surface wave action and those employing transducers.
Representative of the former type of systems are the systems of the following U.S. Pat. Nos.: 4,017,842 to Vineyard, Apr. 12, 1977 (arm with float responds to wave height); 3,778,803 to Jahn, Dec. 11, 1973 (position sensitive switch within floating buoy); 3,504,145 to Layher, Mar. 31, 1970 (buoyant float within buoyant container operates contacts); 3,953,843 to Codina, Apr. 27, 1976 (floating housing has one contact above water and one below); and 3,786,469 to Massaro et al, Jan. 15, 1974 (floating housing with hanging bob contacting conical member). Such systems relying on surface wave action and/or physical quiescence are susceptible to false indicators such as wind or contact with objects.
In one transducer system described in U.S. Pat. No. 3,810,146 to Lieb, May 7, 1974, a transducer is mounted in the wall of a swimming pool and is responsive to ultrasonic signals from special transmitters that must be affixed to children or others that might improperly enter the pool in order to detect their presence. Such a system would fail to detect an unequipped person.
In U.S. Pat. No. 3,969,712 to Butman et al, July 13, 1976, a transducer facing downward from the under surface of a floating housing which also contains circuitry to filter out lower frequency signals received by the transducers, integrate the signals and activate a bell when a threshold is reached. Such a system is susceptible to surface wave action and physical contact with the housing. Also, the transducer orientation likely results in directionality in transducer response. Moreover, the transducer will also be responsive to the natural resonant frequency of the immersed housing. The transducer location also requires that a relatively large area in the housing under surface be sealed against water.